MORRISVILLE, Pa., Nov 4 (Reuters) - Republican presidential hopeful Mitt Romney made a final-stretch campaign stop on Sunday in Pennsylvania, a Democratic-leaning state where polls show he has been gaining on President Barack Obama before Tuesday's election.

"The people of America understand that we are taking back the White House because we are going to win Pennsylvania," Romney told a raucous outdoor rally in the suburbs of Philadelphia, where a crowd estimated by the Secret Service at 30,000 had waited hours to see him on a cold evening.

The mid-Atlantic state offers a trove of 20 Electoral College votes, which would be invaluable to Romney as he tries to amass the 270 needed to win the presidency.

His campaign portrayed the foray into the state as evidence that support for the Republican is growing even in unlikely places. But Obama's team said it shows Romney is desperate as he trailed in polls in some key swing states.

Romney kept to the themes of his stump speech during the Pennsylvania rally, one of many in a frantic last weekend of campaigning, during which he hopscotched in the swing states of New Hampshire, Iowa, Colorado and Ohio. Romney left Morrisville for a stop in Virginia.

He appealed to his supporters to be sure to get out to the polls on Tuesday, but also pitched his argument to independent voters. Pennsylvania has not backed a Republican presidential candidate since 1988.

"You're here because you care about America," he said, urging supporters to talk to undecided friends and neighbors to try to convince them to vote Republican.

Romney praised New Jersey Governor Chris Christie, who has been one of his major supporters but was attacked by many Republicans for praising Obama's response to the massive storm Sandy last week.

Obama advisers dismissed the stop in Morrisville as a desperate last-ditch ploy because Romney has failed to eliminate Obama's lead in most other swing states like Ohio, with less than 48 hours before Election Day.

"They understand that the ... battleground states on which we have been focusing are not working out for them," Obama senior advisor David Axelrod insisted on "Fox News Sunday."

"We're even or ahead in all of them and now they're looking for somewhere, desperately looking for somewhere to try to dislodge some electoral votes to win this election and I can tell you, that's not going to happen," he said.

Axelrod has vowed that he would shave off his signature moustache if Obama loses Pennsylvania - or Minnesota or Michigan, two other Democratic-leaning states where the race has grown tighter in recent weeks.

OBAMA'S POLL LEAD

Most polls show Obama with a lead of 4-6 percentage points in Pennsylvania, a state he won by 10 points over Republican John McCain four years ago. But Obama had led by 10-11 points in opinion polls in September.

Romney's campaign launched ads in Pennsylvania this week, and Obama's responded with spots of its own.

Obama and Romney are essentially tied in most national polls, but the president holds a slight edge in most of the hotly contested battleground states expected to determine who wins the White House.

Romney's campaign said it decided to target Pennsylvania because polls showed the race tightening there after Obama's weak performance in the first presidential debate on Oct. 3.

"We've spent some time in the last few weeks concentrating on expanding the maps," Kevin Madden, a senior campaign aide told reporters on Romney's plane as it headed to Ohio, the most heavily contested swing state, for a rally in Cleveland.

"This is one of those states that came into view right after the first debate and as a result, it presented a great opportunity," he said.

Madden dismissed talk that the visit was too little, too late, noting that Pennsylvania - unlike other states like Ohio and Florida where millions of people have voted already - does not have early voting.

The rally drew a huge crowd, but Romney arrived some 90 minutes after he was expected and hundreds of people streamed out of the rally as he spoke, angry and cold after waiting at a facility with no bathrooms or hot drinks for sale.